Robbie Diack was injured in Friday's defeat by Edinburgh
Ulster number eight Robbie Diack will miss the Heineken Cup games against Harlequins and Stade Francais because of a foot injury.
The South African limped out of Friday's Magners League defeat by Edinburgh and scans have revealed that he has a metatarsal injury.
Diack's injury is currently in plaster and surgery may be required.
With Diack ruled out, Ryan Caldwell is in line to make his Ulster return in Saturday's game against Harlequins.
Caldwell has regained fitness after being out of action for a month because of an ankle injury.